The present invention relates to measuring characteristics of an object such as color, translucence, contrast, texture, roughness and the like. More particularly, the invention relates to optically measuring these characteristics for an object that generates glare when illuminated.
When an object having a smooth, glossy surface is illuminated with light, some of that light usually is reflected in a way that degrades viewing of the object, for example, by creating a bright white spot that appears to be emanating from the object. This bright spot is associated with an optical phenomena referred to as “glare.”
Glare is generated by beams of light from an illumination source being reflected from an object's surface directly along an observation line of an observer, or an optical device, such as an imaging device or a camera. In most cases, glare is a reflected image of the light source itself. FIG. 1 illustrates the interaction of a light beam generated by illumination source 3 with a surface 10 to generate glare, i.e., reflected light beam 8, as viewed by observer O. Incident light beam 2, refracted light beam 4 and reflected light beam 8 interact with surface 10 under Snell's law, which provides that the angle A of incident light beam 2 is equal to the angle B of reflected light beam 8 as referenced to an axis 5 normal to the surface. Refracted light beam 4 interacts with the object. Portions of beam 4, when redirected by interaction to emerge from the object, may be observed by observer O along observation lines 6 to provide useful information, such as color, transparency, texture, etc., about the object. However, where the observer's observation lines 6 coincide with the reflected light beam 8, the observer can only perceive a bright spot appearing to emanate from the surface at point C. This bright spot is referred to as a glare artifact.
Although not a significant problem in casual human observation, glare provides many challenges in photographic and imaging applications because it detracts from captured images and eliminates useful information, e.g., color, contrast, translucency, etc., in locations coinciding with glare artifacts in the images. Accordingly, many conventional imaging devices are configured to manage reflected light beams, particularly light beams reflected from glossy or shiny surfaces, and prevent them from reaching the imaging device to generate glare artifacts in images.
A typical glare-eliminating imaging instrument, shown in FIG. 2, includes a directional light source 3 and an imaging device 12. These components are geometrically positioned to prevent the reflected light beams 8 from reflecting along an observation axis 14 of the imaging device 12, which is at a 45 degree angle from normal to the glossy surface 10. Specifically, the illumination source 3 is configured to project light beams 2 toward the glossy surface 10 along lines normal to the surface. Under Snell's law, the incident light beams 2 generate reflected light beams 8, which reflect toward the light source at an angle normal to the surface. Accordingly, the reflected light beams are not coincident with observation axis 14 nor observation lines 6, and therefore are not detected by the imaging device 12. Thus, no glare is seen by the instrument or generated in resulting images.
Another glare-reducing imaging instrument design uses polarized light to reduce glare artifacts. Specifically, an illumination source projects light polarized at one angle and an imaging device includes a filter to transmit light to the device at a different angle. Reflected light is cross-polarized out from any resulting image.
Although most conventional imaging processes attempt to reduce the impact of glare in captured images, a few actually use it, but only for limited purposes. For example, U.S. Pat. No. 6,222,628 to Corallo measures the intensity of glare from a sample to determine the roughness of a metal surface. In U.S. Pat. No. 5,764,874 to White, the intensity of glare is measured in regions of cigarette paper coated with glue and compared to the measured intensity of glare in regions not coated with glue to ensure that enough glue is applied to the paper. In another example, a specific type of glare—specular reflection from a glossy surface—is used to reconstruct a three-dimensional shape of an object from a two-dimensional image of the object. Specifically, the three-dimensional surface shape of an object is calculated by analyzing locations of specular reflection in either multiple images from multiple viewpoints under one light, or multiple images from a single viewpoint under a different light sources. H. Shultz, Shape Information from Multiple Images of a Specular Surface, IEEE Transactions on Pattern Analysis and Machine Intelligence, 16:195–201 (1994).
Until recently, conventional imaging devices have been designed to reduce the effect of glare on image capture. And even now, the intensity of glare is used only in specific applications to analyze attributes of glare-generating surfaces or extract three-dimensional information from two-dimensional images. Thus, many opportunities exist to exploit the information provided by glare.